Wireless communication systems are widely deployed to provide various types of communication, such as voice and data communications. These systems may be based on a variety of modulation techniques, such as code division multiple access (CDMA) or time division multiple access (TDMA). A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “C.S0002-A Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 cdma2000 High Rate Packet Data Air Interface Specification” (the cdma2000 standard), and (4) some other standards.
Pseudorandom noise (PN) sequences are commonly used in CDMA systems for spreading transmitted data, including transmitted pilot signals. The time required to transmit a single value of the PN sequence is known as a chip, and the rate at which the chips vary is known as the chip rate. CDMA receivers commonly employ rake receivers. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilots from one or more base stations, and two or more multipath demodulators (fingers) for receiving and combining information signals from those base stations.
Inherent in the design of direct sequence CDMA systems is the requirement that a receiver must align its PN sequences to those of a base station. For example, in IS-95, each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations by inserting a unique time offset in the generation of its PN sequences (all base stations are offset by an integer multiple of 64 chips). A subscriber unit communicates with a base station by assigning at least one finger to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. An IS-95 receiver uses one or more searchers to locate the offsets of pilot signals, and hence to use those offsets in assigning fingers for receiving. Since IS-95 systems use a single set of in-phase (I) and quadrature (Q) PN sequences, one method of pilot location is to simply search the entire PN space by correlating an internally generated PN sequence with different offset hypotheses until one or more pilot signals are located.
As the searcher correlates the PN sequence with each offset hypothesis, it records the resulting signal energy. Energy peaks appear for the offset hypotheses that result in recovery of the signal, while other offset hypotheses typically result in little or no signal energy. Multiple energy peaks may result from, for example, echoes produced when signals reflect from buildings and other objects.
In some cases, the reported peaks do not exactly coincide with the actual locations of the energy peaks. The signal propagation delay associated with the distance between a mobile unit and a base station may cause the location of an energy peak to exist at any fractional-chip offset. The searcher typically operates with a resolution of one-half of a PN chip. That is, the searcher tests offset hypotheses that are spaced out by one-half of a PN chip. As a result, the actual location of an energy peak may be at any offset within one-half of a chip from the location reported by the search hardware to the search software. Searching for energy peaks at half-chip intervals may not adequately detect the true location of an energy peak. However, doing so is beneficial to reduce search time, compared to searching at one-eighth chip resolution, for example.
Other systems, such as W-CDMA systems, differentiate base stations using a unique PN code for each, known as a primary scrambling code. The W-CDMA standard defines two Gold code sequences for scrambling the downlink, one for the in-phase component (I) and another for the quadrature (Q). The I and Q PN sequences together are broadcast throughout the cell without data modulation. This broadcast is referred to as the common pilot channel (CPICH). The PN sequences generated are truncated to a length of 38,400 chips. The period of 38,400 chips is referred to as a radio frame. Each radio frame is divided into 15 equal sections referred to as slots.
It is possible to search for W-CDMA base stations in the manner described for IS-95 systems, described above. That is, the entire PN space can be searched offset by offset (38,400 of them) for each of the 512 primary codes. However, this is not practical due to the excessive amount of time such a search would require. Instead, the W-CDMA standard calls for base stations to transmit two additional synchronization channels, the primary and secondary synchronization channels, to assist the subscriber unit in searching efficiently.
For initial acquisition, the three-step W-CDMA search provides a great performance increase, in terms of reduced search time, over the impractical alternative of searching the entire PN space for each scrambling code.
Search time is an important metric in determining the quality of a CDMA system. Decreased search time implies that searches can be done more frequently. As such, a subscriber unit can locate and access the best available cell more often, resulting in better signal transmission and reception, often at reduced transmission power levels by both the base station and the subscriber unit. This, in turn, increases the capacity of the CDMA system, either in terms of support for an increased number of users, higher transmission rates, or both. Decreased search time is also advantageous when a subscriber unit is in idle mode. In idle mode, a subscriber unit is not actively transmitting or receiving voice or data, but is periodically monitoring the system. In idle mode, the subscriber unit can remain in a low power state when it is not monitoring. Reduced search time allows the subscriber unit to spend less time monitoring, and more time in the low power state, thus reducing power consumption and increasing standby time.